1. Field of the Invention
The present invention relates generally to a Viterbi decoder of a mobile communication system, and in particular, to an apparatus and a method for improving the performance of a decoder by improving a decoding speed during error correction of a control signal in Long Term Evolution (LTE).
2. Description of the Related Art
A mobile communication system is being standardized and having as one of its core features efficient cooperation between a wired communication network and a wireless communication network as in integrated service as well as simple wireless communication service. Therefore, development of technology for transmitting high capacity data approaching the capacity of a wired communication network in a wireless communication network is required.
As a high-speed high-capacity communication system that can process and transmit various information such as images, wired data, etc. as well as a voice-oriented service is required, an appropriate channel coding method that can improve system performance by increasing a system transmission efficiency is an essential element. Also, in a wireless channel environment existing in a mobile communication system, unlike a wired channel environment, errors are generated due to various factors such as multipath interference, shadowing, radio wave attenuation, a time-varying noise, interference and fading, etc., so that information loss can occur. The information loss acts as a factor that deteriorates the performance of the entire mobile communication system by causing serious distortion to an actual transmission signal.
Generally, the reliability of a system is raised by using various error control techniques depending on the character of a channel in order to reduce this information loss. One of the fundamental methods of these error control techniques is to use an error correction code. A general mobile communication system uses a Viterbi decoder in order to correct an error associated with a channel noise. A frequently used method for correcting an error associated with channel noise is a decoding technique of searching for a Maximum Likelihood (ML) path regarding a survivor path through a traceback operation with respect to data coded with a convolution code.
The above-described decoding technique through the traceback operation is a method of searching for the ML path by tracing back, from back to forth, survivor path information created after an Add Compare Select (ACS) operation is performed. A decoding technique using a traceback operation of a Window mode is described as an example. When performing a traceback operation, a received signal to be decoded is decoded using a characteristic that even when an initial state value is arbitrarily set, the set value converges to the ML path when passing through sections to some degree. When the length of a received signal to be decoded is long, the decoder cuts the length of the received signal with a predetermined length, performs a training section on a length corresponding to a window mode of a predetermined size, and performs a decoding operation from a section after the training section.
FIG. 1 is a timing diagram illustrating an operation of a decoder through a traceback operation of a general mobile communication system.
Referring to FIG. 1, when receiving an input signal, the decoder performs an Add Compare Select (ACS) operation, and performs a traceback operation in the form of a window mode. At this point, the decoder performs the traceback operation using two windows in order to perform a successive decoding operation.
First, a traceback operation 100 for TB (B) and TB (A) is performed using a first Window A, and a traceback operation 110 for TB (C) and TB (B) is performed using a second Window B, so that traceback operations for the windows are performed in turns. Here, TB (B) of Window A represents a training section, and TB (A) represents a decoding section.
Accordingly, the decoder outputs, in turn, results of tracebacks of Window A and the Window B. That is, the decoder outputs (101) a result of Window A (100), and outputs (111) a result of Window B (110). After that, the decoder outputs (121) a result of Window A (100), and outputs a result of Window B again.
At this point, an initial output for an initial input signal is different by three sections, so that when an input signal corresponding to a fourth section is generated, the decoder outputs a result of the initial input.
FIG. 2 is a timing diagram illustrating another operation of a decoder through a traceback operation of a general mobile communication system.
Referring to FIG. 2, the operation process of the decoder one in which a tail biting code is used, in which a received signal is lengthened and the lengthened signal is decoded.
In other words, the decoder generates a lengthened signal called IN (T) IN (A) by attaching IN (T)(201) obtained by copying an input signal IN (A)(203) to the input signal IN (A)(203), and performs a decoding operation on the signal IN (T) IN (A). At this point, the decoder twice performs the traceback operations on the input signal IN (A) and outputs a result thereof Before outputting the result, the decoder determines whether there exists an error through a Cyclic Redundancy Check (CRC) check. When an error is generated during the first traceback operation, the decoder performs the second traceback operation.
The above-described decoder improves a decoding performance by performing an iterative traceback operation. However, the decoder requires a block called Last In First Out (LIFO) block in order to correct information output backward while the traceback operation is performed, and performs a CRC check after storing a decoded bit in the LIFO. This process creates an operation delay in a CRC check process.